In present communication systems, 2G and 3G communication systems coexist. Due to the mobility of a user terminal, it is possible for the user terminal to roam across two communication systems during a call process. Thus a handover between 2G and 3G communication systems is necessary to avoid interrupting the conversation of the user terminal moving between different communication systems.
In handing over a 3G user terminal from a 3G communication system to a 2G communication system, a mobile switching center (MSC) server in the 3G communication system will send handover-related information such as radio frequency capability (RFC) of the user terminal and a cell ID and so on to the 2G communication system so that the 2G communication system can handover the 3G user terminal successfully based on the received information.
Taking RFC as an example, in 2G communication systems, e.g. Global System for Mobile Communications (GSM) 900 or Data Cellular System (DCS) 1800, the value of RFC ranges from 0 to 4, while in 3G systems, such as Universal Mobile Telecommunications System (UMTS), 7 is designated as the value of RFC. Since 2G systems can not identify user terminals with 7 as RFC and 3G systems do not accept user terminals with RFC ranging from 0 to 4, it is impossible to hand over 3G mobile stations between 3G and 2G systems.
Similarly, in 2G communication systems such as GSM systems, the valid value of a cell ID identifier in a cell ID can only be one of Cell Global Identity (CGI) (0000 (binary)), Location Area Code Cell Identity (LAC_CI) (0001 (binary)) and CI (0010 (binary)). However, in 3G communication systems such as UMTS, the valid value of the cell ID identifier in UMTS can be one of CGI (0000 (binary)), LAC_CI (0001 (binary)), CI (0010 (binary)), Location Area Identity (LAI) (0100 (binary)), Location Area Code (LAC) (0101 (binary)), LAI_Radio Network Controller ID (RNC ID) (1000 (binary)), RNC ID (1001 (binary)), LAC_RNC ID (1010 (binary)) and Service Area Identifier (SAI) (1011 (binary)).
According to 3GPP TS 25413, when an RNC initiates a handover from a UMTS system to a GSM system, a serving cell ID (Source ID) takes SAI as its value, and the corresponding cell ID identifier is 1011 (binary) (SAI). In this way, for a mobile switching center (MSC) which has not been upgraded in a 2G communication system, a cell ID identifier with the value of 1011 cannot be dealt with, which leads to a failure in handover. In addition, when the cell ID identifier takes CGI as its value, the mobile network code (MNC) of the UMTS network may be different from the MNC of the existing 2G network MNC, which also leads to a failure in handover because the MSC/base station controller (BSC) in the 2G network cannot identify the serving cell ID.
It can be seen that defects of the existing method for inter-system handover of 3G user terminals lies in that the difference between the handover-related information supported by 3G systems and by2G systems leads to unsuccessful handover.